The problem is, if (s)he is using the official packages, he will get the
updates of the mainline channel, thus differing more and more from the
stable channel which is supposed to be cheaper on features but with
usable
ones.

I was anticipating such a compatibility problem to be fixed in feature stable
but so far its looking like we will have to bite the bullet and move to mainline.
Would I be correct here? It seems for our case at least, feature stable HTTP2 is
not stable for production use at this time.

Correct. It has been stated multiple times that mainline is the
suggested branch for HTTP2 use and that statement is still true.

I’m beginning to hear many reports now of Safari and iOS users hitting
connection issues on several websites, not just around upload but any
arbitrary POST it seems.
Building from source is a no go too, and we’ve always stayed on the
feature stable branch as it’s easier to maintain since we don’t need to
worry about additional features causing issues.

I was anticipating such a compatibility problem to be fixed in feature
stable but so far it’s looking like we will have to bite the bullet and
move to mainline.
Would I be correct here? It seems for our case at least, feature stable
HTTP2 is not stable for production use at this time.